This invention relates to a radio signal receiver that receives radio signals and to the gain-control method thereof, and more particularly, to a radio signal receiver and gain-control method thereof that converts a radio received signal to a baseband received signal, and converts that baseband received signal to a digital signal, and performs specified baseband processing based on that digital signal.
A radio signal receiver receives a radio signal from an antenna, and after converting that received signal to a baseband signal, converts the baseband signal to a digital signal, and performs baseband processing such as demodulation, reception-quality measurement, channel estimation/compensation, error-correction and decoding of that digital signal. FIG. 7 is a drawing showing the construction of a prior radio signal receiver. A radio signal is received by way of an antenna and input to a RF-receiving unit 2. The RF-receiving unit 2 comprises: a high-frequency amplifier 2a; a bandpass filter 2b that restricts the bandwidth, and a frequency-conversion unit 2c that performs frequency conversion processing of the RF signal and converts this RF signal to a baseband signal. An orthogonal-wave-detection unit 3 performs orthogonal-wave detection on the received baseband signal, and outputs two orthogonal signal components I, Q that are orthogonal to each other, variable-gain amplifiers (amps) 4a, 4b amplify the orthogonal signal components I, Q, respectively, and input the results to AD-conversion units 5a, 5b, where those AD-conversion units 5a, 5b convert the input analog received signals to digital signals and input those signals to a baseband-processing unit 6 and average-level-measurement unit 7. The baseband-processing unit 6 performs the aforementioned processing, and the average-level-measurement unit 7 calculates the power from I2+Q2, and outputs the average value over a specified period. An amp-gain-adjustment unit 8 sets the gain so that the level of the input to the baseband-processing unit is constant, and inputs the result to the amps 4a, 4b. The average-level-measurement unit 7 and amp-gain-adjustment unit 8 form an auto-gain-control unit (AGC circuit) 9.
As described above, the prior AGC (Automatic Gain Control) circuit 9 generally measures the reception power after AD conversion, and controls the gain of the amps 4a, 4b based on the result. However, with this construction, it is difficult to handle reception of a burst signal that occurs in packet communication. Here, reception of a burst signal includes burst-like changes in the reception level as a result of adaptive-modulation control or scheduling control.
FIG. 8 is a drawing that explains conventional automatic gain control. When there is a low incoming signal, the average level of the incoming signal becomes low and is expressed by ‘b’ bits. At this time, assuming that the upper limit is larger than the average reception level by the level corresponding to ‘a’, bits, the width of the range of the low incoming signal becomes (a+b) bits. On the other hand, when there is a high incoming signal, the average level becomes high and is expressed by (b+c) bits. Here, ‘c’ bits is the difference between the average level of the high incoming signal and the average level of the low incoming signal. When the incoming signal is a high incoming signal, assuming that the upper limit is larger than the average reception level by the level corresponding to ‘a’ bits, the width of the range of the high incoming signal becomes (a+b) bits.
In this conventional automatic gain control, the average reception level is measured after AD conversion, and the gain of the amp is controlled so that the average level corresponds to ‘b’ bits. For example, the gain of the amp is controlled so that the binary value of “b” bits each of which is “1” becomes the measured average level. By performing AGC in this way, when there is a high incoming signal and when there is a low incoming signal the width of the range is (a+b) bits, so it is sufficient that the number of output bits of the AD-conversion units 5a, 5b is (a+b) bits.
However, there is a difference (maximum of ‘c’ bits) between the average level of a high incoming signal and low incoming signal. Due to this difference, when the average level of the incoming signal becomes low by reception of a burst signal during period of a high incoming signal, or when the average level of the incoming signal becomes high by reception of a burst signal during period of a low incoming signal, the reception level greatly changes from the average level of ‘b’ bits, and this becomes a problem. That problem will be explained below.
As shown in (A) of FIG. 9, in order to cope with the case where the average level of the incoming signal becomes low by reception of a burst signal during period of a high incoming signal, it is necessary to increase the average level to the level of (b+c) bits. In other words, it is necessary to make the average level be level of (b+c) bits and make the upper level limit be a level ‘a’ bits higher than the average level. On the other hand, as shown in (B) of FIG. 9, in order to cope with the case where the average level of the incoming signal becomes high by reception of a burst signal during period of a low incoming signal, it is necessary to make the average level be the level of ‘b’ bits, and make the upper level limit (a+c) bits higher than the average level.
The receiver does not know whether the receiving state is in a high-incoming-signal state, or in a low-incoming-signal state. Therefore, in order to correspond to both of the aforementioned states, or in other words, in order to express the level of the received signal without distortion, it is necessary to specify a high level of (a+c) bits as the upper limit level when the maximum average level is a level of (b+c) bits. As a result, it is necessary that the total number of bits output from the AD-conversion units 5a, 5b be (a+c)+(b+c) bits, or in other words, a large (a+b+2c) bits.
From the above, when taking into consideration reception of a burst signal, the necessary number of bits for the AD-conversion units becomes large. As the necessary number of bits for the AD-conversion units becomes large, there is a problem in that the operation speed decreases, and hardware becomes large. Also, there is a limit on the maximum number of bits for AD-conversion units, and as the number of bits (a+b+2c) becomes larger, there is a possibility that there will be no AD-conversion unit that can satisfy that number of bits.
Various prior AGC circuits have been proposed (for example, see JP2006-135617A), however, no AGC circuit has been proposed that decreases the number of bits required for the AD-conversion units even when a burst signal is received.